1. Field of Invention
This invention relates generally to exhaust gas recirculation (EGR) systems for controlling emissions in vehicles and more particularly to a closed loop exhaust gas recirculation system which senses the actual operating parameters of absolute manifold pressure (MAP), engine speed (RPM), and throttle position (.theta.), or air flow (AF), and accurately controls EGR using a comparison of uncompensated, or preferably pressure compensated, values of actual absolute manifold pressure MAP with preprogramed optimal values of absolute pressure MAP.sub.0 stored in a look-up memory table.
2. Statement of the Prior Art
It is generally recognized that the production of noxious oxides of nitrogen (NO.sub.x) which pollute the atmosphere are undesirable and in many cases are controlled by limits established by local, state and federal governmental regulations. The formation of NO.sub.x constituents in the exhaust gas products of an internal combustion engine must therefore be eliminated, minimized or at least maintained below some predetermined limit.
It is generally understood that the presence of NO.sub.x in the exhaust of internal combustion engines is determined by combustion temperature and pressure. An increase in combustion temperature causes an increase in the amount of NO.sub.x present in the engine exhaust. It is, therefore, desirable to control the combustion temperature in order to limit the amount of NO.sub.x present in the exhaust of an internal combustion engine.
One method suggested by the prior art for limiting or controlling the combustion temperature has been to recirculate a portion of the exhaust gas back to the engine air intake. Since the exhaust gas is low in oxygen, this will result in a richer combustion mixture which will burn at a lower temperature. The lower combustion temperature will, in turn, reduce the amounts of NO.sub.x produced during combustion.
Similarly, it has, until recently, been common practice to run an internal combustion engine at or near a spark timing which produces maximum peak combustion pressures. Unfortunately, however, at or near peak combustion pressures, unacceptably high levels of NO.sub.x are produced in the combustion chambers when the engine operates at or near spark timings which produce maximum peak combustion pressures. In order to inhibit the formation and emission of NO.sub.x it is therefore desirable to limit the peak combustion pressure to a selected value.
One technique suggested by the prior art for limiting combustion pressure involves the recirculation of exhaust gases through the induction passage of the combustion chamber since it is well-known that an increase in recirculation of exhaust gases will reduce peak combustion pressure and thus the attendant levels of undesirable NO.sub.x. Similar results may be achieved by retarding the spark.
Therefore, it is generally well-known that the formation of undesirable oxides of nitrogen may be reduced by recirculating a portion of the exhaust gas back to the engine air/fuel intake passage so as to dilute the incoming air/fuel mixture with inert N.sub.2, H.sub.2 O, and CO.sub.2. The molar specific heat of these gases and especially of CO.sub.2 absorbs substantial thermal energy so as to lower peak cycle temperatures and/or pressures to levels conducive to reducing NO.sub.x formation.
While NO.sub.x formation is known to decrease as the exhaust gas recirculation (EGR) flow increases to where it represents about twenty percent of the exhaust gas constituents, it is also known that this is accompanied by a deterioration in engine performance including, but not limited to an increase in the engine roughness with increasing EGR. Therefore, one factor limiting the magnitude of EGR is the magnitude of EGR-induced performance deterioration or roughness that can be tolerated before vehicle drivability becomes unacceptable.
Most of the prior art attempts at solving these problems have employed various mechanical schemes for directly controlling the position of the EGR control valve which may be operated by sensing a single parameter such as throttle position, intake manifold pressure, exhaust back pressure, the air/fuel ratio, etc.
Such prior attempts to control EGR by sensing and shaping signals indicative of a single parameter of engine performance or sensing engine flow as a function of venturi vacuum or exhaust back pressure are not conducive to accuracy or programability.
Closed loop control systems for controlling various parameters of an internal combustion engine are known in the art, as are the above-identified EGR control systems, even though the prior art does not include many EGR control systems employing a closed loop mode of operation. U.S. Pat. No. 3,872,846 issued to L. B. Taplin et al on Mar. 25, 1975 for an Exhaust Gas Recirculation (EGR) Internal Combustion Engine Roughness Control System and is assigned to the assignee of the present invention. This patent teaches a closed loop internal combustion engine control system provided for controlling the exhaust gas recirculation flow so as to regulate engine roughness at a predetermined level. The closed loop control system of this patent receives input signals indicative of the engine roughness and generates therefrom an EGR valve command signal for varying the position of an EGR valve so as to effect the maximum possible EGR flow compatible with a predetermined maximum level of permissible engine roughness.
It is desirable to be able to control the generation and emission of noxious oxides of nitrogen during all modes of engine operation without causing vehicle drivability to reach an unacceptable level. It is also desirable to be able to accurately and programatically control NO.sub.x formation in a manner so as to allow a trade off between vehicle drivability and NO.sub.x formation, to suit the requirements of a given application.
It is, therefore, an object of the present invention to provide a new and improved closed loop control system for reducing the formation of certain exhaust constituents of an internal combustion engine.
It is another object of the present invention to provide a closed loop control system for preventing deterioration of engine performance below a predetermined level, for regulating engine roughness, and the like.
It is another object of the present invention to provide a method and apparatus employing a closed loop egr control system which will provide improved accuracy over the entire range of engine operating conditions, greater programability of the amount of EGR for various conditions of speed and load, and improved control during transient modes of operation while simultaneously allowing a tighter control of NO.sub.x emissions.
It is yet another object of the present invention to provide an economical, highly reliable, programable method and apparatus for accurately maintaining a predetermined balance between NO.sub.x formation and vehicle drivability.
These and other objects and advantages of the present invention will be accomplished by the method and apparatus for the closed-loop control of EGR utilizing manifold pressure, engine speed, and either throttle position or air flow. For greater accuracy, compensation for variations in altitude and/or ambient barometric pressure are also provided.